This invention relates to the preparation of very fine glass powder of high purity having a mean particle size d.sub.50 of .ltoreq.10 .mu.m by wet-grinding in the presence of grinding elements.
High-purity glass powders are required, in particular, as fillers for plastics employed in the dental sector, for example, dental fillings. For glass powders of this type, mean particle diameters d.sub.50 of at most 10 .mu.m, preferably &lt;5 .mu.m, in particular .ltoreq.3 .mu.m are required since the mechanical properties such as polishability and abrasion resistance, are improved with increasing fineness (decreasing particle diameter). Excessively large glass particles (&lt;10 .mu.m) produce a rough surface in the cured plastic or break out and leave holes and sharp edges. The refractive index of the glass powder must agree very closely with that of the plastic in order to achieve high transparency and translucency of the filled plastic. If the glass powder contains, for example, coloring particles or particles having different refractive indices, the translucency and transparency and possibly also the color of the filled plastic are impaired, so that the plastic can frequently only be used with considerable restrictions, if at all.
Glass powders are prepared by grinding. The grinding processes hitherto have the disadvantages of, in some cases, high energy consumption for the grinding, long grinding times for fine particle sizes and high abrasion of grinding pebbles and the mill wall. The abrasion particles impair the transparency and translucency of the filled plastic and make the production of very pale tooth colors difficult.
The conventional dry-grinding processes are at the limit of their performance for these small particle sizes, require long grinding times and generally require an additional air separator for classifying the grinding material. Abrasion of the grinding elements, wear of the grinding container or of the air separator and the energy consumption are so high that these grinding processes are unsuitable for the preparation of very fine glass powders.
Although wet-grinding processes using water yield fine particle sizes in a shorter time than do dry-grinding processes, the grinding elements are still subject to considerable abrasion and a particular disadvantage is that numerous agglomerates, i.e., very solid clusters of powder particles, which act in a similar way to large individual particles and dramatically impair the properties of the filled plastic, form from the grinding slurry on drying. If, by contrast, the grinding is carried out in the presence of organic liquids in which agglomeration is substantially suppressed on drying (for example, low-boiling hydrocarbons), the grinding times are considerably extended, the amount of grinding abrasion increases correspondingly and additional safety precautions for example, explosion protection, become necessary.